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Homework Help: Energy sequence

  1. Oct 5, 2009 #1
    Hi, I'm sorry if I post this question at the wrong section. I have a question regarding the energy sequence.

    Qn: find the energy of the sequence x(n) = (1/3)^n u(n-1) + (1/2)^(n-1) u(n-2)

    My Approach:
    1). Energy = (sum from negative infinity to infinity) of |x(n)|^2
    2). I know that if a sequence is in the form of x(n) = a r^n u(n), for example if x(n) = (1/3)^n then we can compute the energy simply by E = (sum from 0 to infinity) of (1/3)^n = 1/(1-1/3) = 3/2.

    My question is:
    1). What if it is x(n) = a r^(n-1) instead of only x(n) = a r^n
    2). And when u(n-1) instead of u(n)
    *what I knew for u(n-1) is that we need to consider the sum from 1 to infinity instead of from 0 to infinity(like when it's only u(n)), but then after that I'm not sure on how to start the calculation at all. I'm so confused about this.

    Does anyone can help me with this?

    Thank you!
  2. jcsd
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